"Endothelial Supporting" Function of Platelets: Effect(s) of platelets on integrity of small vessel endothelium will be studied in the isolated, perfused dog kidney. Kinetics of platelet "membrane" and "cytoplasmic" platelet labels will be compared in rats, dogs, and man, with and without administration of drugs inhibiting platelet function. Incorporation of platelet "membrane" labels into endothelium of thrombocytopenic rats will be assessed by autoradiography after transfusion of tagged platelets. Functional and structural changes occurring as platelets "age" in the circulation will be further characterized. Pathogenesis of Immune Purpuras; Characterization of Platelet Auto and Iso-Antigens: Cellular and humoral immunity to autologous platelets will be studied before, during, and after treatment of patients with "idiopathic" thrombocytopenic purpura to improve methods of diagnosing, treating, and predicting prognosis in this disorder. The pathogenesis of post-transfusion purpura will be further investigated. Auto and isoantigens of platelets will be isolated, characterized, purified in large quantities, and applied to the surface of erythrocytes. The coated erythrocytes will be studied for their usefulness as indicator cells for detection of platelet antibodies using the hemagglutinating Auto-Analyzer. Molecular Basis of the Platelet "Storage Lesion.": Biochemical changes occurring in platelets during storage will be characterized, with particular attention to lipids, cyclic nucleotides, and prostaglandin synthetic systems. Findings will be applied to the development of improved methods of short-term platelet preservation.